Friday, February 6, 2009

Substantially Identical Outfalls - Substantially Identical Effluents

One option potentially available to dischargers required to conduct stormwater monitoring is the use of "Substantially Identical" outfalls. This is particularly useful to facilities with many outfalls that need to be monitored.

The selection of substantially identical outfalls does not have to be "pre-approved" in many cases. However, the risk to dischargers is that if the permitting agency does not agree with the determination of substantially identical outfalls, the discharger may be subject to enforcement action. Therefore, careful consideration and documentation that the selected identical outfalls are valid is important.

The criteria used to determine if the "Substantially Identical" option applies will change from State-to-State. Generally, the factors used can include:

  • Location ;
  • Industrial activities conducted in the drainage area of each outfall;
  • Control measures implemented in the drainage area of each outfall;
  • Materials used or stored;
  • Runoff coefficient of the drainage areas.

Caltha LLP assists dischargers nationwide in developing and implementing cost effective stormwater monitoring programs, including determination and documentation of substantially identical outfalls. To request further information, go to SWPPP and SPCC website.

For further information contact Caltha LLP at
Caltha LLP Website

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